The common conception was that an asteroid strike contributed to the extinction to dinosaurs, but unbeknownst to all it was not a giant impact that killed them off; it was actually a world wide plague that destroyed them.

The plague has now returned in modern day. The plague spreads world wide and is deadly for humans, though certain races are more immune than others.

My question is this; what is a scientifically realistic way this could happen? If a plague did indeed kill the dinosaurs how could the plague:

A) Become stagnant for millions of years and disappear from history?

B) Only to return again in modern day to wreck havoc for a second time. Whether it be by the hands of a scientist or through some other event. It returns and hits hard.

C) Be deadly to humans despite being tens of millions of years old?

Trivia

Simple facts about the plague:

It is only deadly for certain living things, unfortunately humans
make the list.

It affects cell motility.

Roughly 30% of individuals have some degree of immunity towards it with race playing a factor. No one has complete immunity (at least not naturally).

It spreads fast.

Note:While I was originally only looking for hard science answers, I am now accepting any explanations that are at least within the realm of realism. Hard science answers are still welcomed though if anyone has anything to contribute. But I've come to understand that the nature of my question makes hard science quite limited.

$\begingroup$Nice question, I like a tough challenge without resorting to handwaving or magic.$\endgroup$
– Measure of despare.Jan 16 at 21:16

5

$\begingroup$I don't know that hard science can apply. Species jumping plagues happen but for it to be that lethal to something so biologically different...Hard science means verifiable numbers in the answer and I think this might be far too speculative to apply to this. Will upvote!$\endgroup$
– Erin ThursbyJan 16 at 21:33

$\begingroup$@ErinThursby I'm willing to become more lenient if enough time passes without any answers. But for now I'm holding out on the hope that someone can think of something plausible.$\endgroup$
– NobleJan 16 at 21:36

$\begingroup$I think Erin is right. Something biological will almost never make that big of a genetic jump. Also, some strains of microbes can survive dormant for years, but at this time frame, it is not realistic, and if it was not dormant, it could have evolved so much in that time frame, that they would not really be at all the same thing at all. The "most" plausible way I see for this to happen would be a time-traveling mishap gone wrong, but that is a debatable option for hard-science.$\endgroup$
– NosajimikiJan 16 at 22:02

2

$\begingroup$Note about "race playing a factor". To the (very very limited) extent that human races have a real biological meaning, you have two possibilities: if you want to make Europoids (a.k.a. white people or, in America, "Caucasians") a race, then sub-Saharan Africa is home to a few dozen such races; if you want to make sub-Saharan Africans (a.k.a. "black people") a race, then this race comprises all mankind.$\endgroup$
– AlexPJan 16 at 22:38

20 Answers
20

I think that is pretty straight forward: A plague that is that deadly for the dinosaurs is a huge evolutionary pressure. Therefore most dinosaur species died out, but the line that later became birds had some key mutations so that the plague was no longer deadly to them, but merely a nuisance (such as a cold for humans). Note that there is evolutionary pressure on the plague as well, not to be too deadly, therefore there are examples that deadly diseases get less virulent over time.

At some point, the plague crosses the species barrier and wreaks havoc among humans.

This is also not unheard of. Ebola seems to be an equivalent of a common cold among flying foxes who are adapted to it. It is quite deadly for humans.

This scenario is entirely plausible. I just don‘t think that the plague would actially kill all the dinosaurs because of the evolutionary pressure to not kill its hosts too fast. But possible: Yes.

$\begingroup$@jamesof: At least in popular science it is also sometimes portrayed as „Birds are dinosaurs“. The point is that birds are direct descendants of dinosaours and they could carry forward a dinosaur disease.$\endgroup$
– Soeren D.Jan 17 at 4:38

2

$\begingroup$The point is: At the time of the dino extinction, dinos and birds were closely enough related that birds got infected, but not killed.$\endgroup$
– Stig HemmerJan 17 at 9:50

1

$\begingroup$@frarugi87 Maybe the dinosaurs were aliens. Or the humans are...$\endgroup$
– Soeren D.Jan 17 at 11:04

The etiologic agent of plague, Yersinia pestis, is a gram-negative coccobacillus and a facultative intracellular pathogen. Y. pestis exhibited the highest overall mortality rate of any infectious disease from its earliest recorded emergence through 1941. During 2010–2015, a mean of 650 cases were reported globally each year, with a case fatality rate of 23%–41% (depending on manifestation as bubonic, pneumonic, or septicemic plague), rising to 66%–100% when adequate medical care was not promptly received. Y. pestis primarily infects small ground-dwelling mammals, specifically of the taxonomic order Rodentia, but maintains high spillover potential to other vertebrates, including humans, caused by its high virulence and fleaborne transmission. Epizootic plague is typically vectored by multiple flea species and is transmitted within and between meta-populations of hosts by flea bites.

Plague ecology is characterized by sporadic epizootics, followed by 2–5-year cryptic dormancy periods. Despite much information on epizootic transmission mechanisms, little is known about the origin of re-emergent plague cases in wild animal populations. Plague among wild animals commonly re-emerges in plague foci after multiple years of inactivity, despite ongoing biosurveillance and attempts at detection during interepizootic periods. The existence of environmental plague reservoirs has been theorized for >80 years. Various avenues of recent research suggest that soil-dwelling amebae may be competent environmental reservoirs of Y. pestis. Amebae are a taxonomically diverse group of phagocytic organisms residing in every major lineage of eukaryotes. Amebae are pervasive in soil and water environments and are recognized for their ability to harbor pathogens that drastically affect ecologic communities. Free-living amebae cycle between 2 distinct life-states: trophozoites, an active, mobile, feeding state; and cysts or spores, a robust dormant state induced in part by adverse environmental conditions.

That's a long-winded and technically precise way of saying that while the Black Death was spread via rats and fleas — the problem is that it lays dormant in soil and water, waiting for the right combination of climate and ecology to become active again. This is why it keeps flaring up all over the world.

It is reasonable and believable that the pathogen that killed the dinosaurs in your story, a pathogen that would have flourished in predominantly cold-blooded critters living in a Mesozoic climate (and not being dissimilar to Y. pestis) is waiting for a big old lizard and the same climate to coincide again. It's in the soil. It's in the water. We've just never had a reason to look for it.

Salmonella are commonly found in all types of reptiles and can spread from reptiles to humans when something contaminated with reptile faeces is placed in the mouth. (Source)

So, Lizard meets warm, moist, somewhat old-fashioned climate, voodoo plague rears its ugly head and bonds with the salmonella... lizard poops right on top of this amazing Cacao plant that happens to benefit from the Mesozoic climate, bean is picked and (say it ain't so!) not cleaned very well... and served as your favorite chocolate confection at Walmart.

$\begingroup$Dinosaurs aren't lizards. I believe most (if not all) were warm-blooded, and if you want to think of them in simplistic modern terms, you should consider them birds. And of course humans get diseases from birds (Influenza) worldwide every year.$\endgroup$
– T.E.D.Jan 17 at 15:32

$\begingroup$@T.E.D., there is no consensus on that matter. In fact, some scientists argue that they are naturally cold-blooded, but their size created a stabilizing effect that results in some of the same benefits of being warm-blooded. This answer provides an insight from one point of view. I'm sure the OP would love another.$\endgroup$
– JBHJan 17 at 15:51

1

$\begingroup$What there is consensus on is that they are far closer related to modern birds than they are to modern lizards.$\endgroup$
– T.E.D.Jan 17 at 15:59

$\begingroup$@T.E.D., that doesn't make them birds. As I said, mine is one point of view. If you have another, the OP will benefit from hearing from you.$\endgroup$
– JBHJan 17 at 16:50

$\begingroup$This answer is honestly close enough that it wouldn't be worth anyone's time for me to write something else. I just have trouble clicking that uparrow over this one issue. If you feel strongly about the lizard comparison though, and feel you can back it up, no I wouldn't change it your shoes then either.$\endgroup$
– T.E.D.Jan 17 at 18:49

If we accept the panspermia theory, that life evolved somewhere else and was brought to earth in some way, possibly in meteoric ice for example, we have our vector.

The pathogen that killed the dinosaurs was carried to earth on a meteor or comet that impacted around the time they died out.

Another impact, or someone digging up pieces of that rock or finding infected dinosaur tissue, releases the pathogen back into the environment, where it quickly finds a new host, human beings, wreaking havoc and killing millions, if not billions, in short order.

This is in fact a real concern of scientists hunting for for example mammoth tissue in Siberia, that those remains contain dormant pathogens that could lead to outbreaks (though how serious it's taken I don't know).

$\begingroup$Alternatively, scientists also dig holes and extract ice carrots to get an idea of prehistoric climate, where one can have similar concerns about releasing a plague trapped in ice.$\endgroup$
– AmiralPatateJan 17 at 8:35

$\begingroup$Ok...but why would we accept that theory? I mean, its fun and all, but not particularly useful scientifically as a theory, and it fails Occam's Razor miserably. (In fact, I think it isn't even necessary in this answer. A melting glacier or someone picking up the wrong rock in Antartica would do the same job much more plausibly)$\endgroup$
– T.E.D.Jan 17 at 15:36

The pathogen was preserved in permafrost Tundra which used to be jungle in the Mesozoic. Infected dinosaur remains got buried and preserved. The remains are now thawing together with the former permafrost soil due to climate change and release the pathogen, perhaps because the remains get eaten by scavenging animals (it's like our deep frozen chicken nuggets that need to be eaten when the freezer fails).

As an aside, the permafrost is melting because we are burning all the coal that used to be that very jungle. That releases the carbon sequestered in it to re-create the CO2 levels of the Mesozoic with all the side effects, good (dinosaurs!) and bad (no humans!).

The bacteria needs to be present but harmless. What people don't know is that the bacteria has environmental triggers linked to the CO2 levels in the air which cause it change and produce toxins, a bit like algae.

I'm not sure how far departed from a scientifically accurate story you want to go, but how about this - a time-travel factor. (I'm sure combining other hard-sci-fi answers from related questions might help give this more of an air of realism?)

Plot twist: The pathogen was developed by an extra terrestrial species (even more interestingly, a species that has its roots from modern humanity, but has long since forgotten those roots), many centuries in the future.

In the middle of the 25th-or-so century, a conflict between warring planets resulted in one party developing a pathogen which targets a large percentage of bio-matter which evolved in certain conditions only found on Earth.

This party decides the most effective way to resolve the conflict would be to target the humans at the point just before they became a space-faring species. However time travel is unpredictable, and the first salvo of the virus missed it's target by an order of a few million years - materializing at the end of the Cretaceous–Paleogene period, and thus triggering a mass extinction event.

Realizing that the weapon missed it's mark, the species sends another payload, this time hitting close to the desired time frame. However the first space faring humans have now left the outer solar system, and the war is destined to happen anyway.

Apologies if this is completely departed from your original concept, but I thought I'd jot it down in case it helps someone.

So, there are two main types of plague (there are a few others but these are the big ones):

Viral

Bacterial

Viral

A Virus tends to be well adapted to a specific host and because it hijacks the hosts DNA and cell machinery in order to replicate it it tends not to be good at crossing species. When it does cross though it tends not to be so well adapted so ironically a non-native virus that does manage to replicate is often the most dangerous type.

This is what gives us "bird flu" as being worse than regular flu.

It's unlikely that a dinosaur virus would be able to infect and spread in humans, we're just so different. If it did happen though it would potentially be very dangerous.

Researchers from the University of Arizona, Tuscon, placed a tracer virus on commonly touched objects such as a doorknob or tabletop. At multiple time intervals – from two to eight hours – the researchers sampled a range of surfaces including light switches, bed rails, countertops, sink tap handles, and push buttons. They found that between 40 and 60 percent of the surfaces were contaminated within two to four hours.

“If we placed a tracer virus on the push plate to an office building, it ended up on almost 50 percent of the high-touch surfaces and office workers’ hands within four hours,” says study author and microbiologist Charles Gerba, PhD. “In the case of the hotel, we placed the virus on the nightstand in one room, and it was spread to the next four rooms by the maid during cleaning.”

Pros:

If it did happen could plausibly be very deadly

Especially if airborne, they spread fast

Can survive for a very long time dormant in the correct conditions

Cons

Unlikely to be able to replicate in humans

Unlikely to be able to spread between humans even if one got infected (see recent Bird Flu cases which have fizzled out).

Bacterial

Bacteria have the advantage of caring far less about the species they are infecting, however since they have to replicate themselves rather than hijacking host cells they tend to spread more slowly and find it harder to survive outside the host environment for a long time.

The good (bad) news is that microbes can survive a long time as this study shows.
Microbes can survive trapped inside ice crystals, under 3 kilometres of snow, for more than 100,000 years, a study back in 2007 suggested.

Thus, virtually any microbe can remain alive in solid ice, resisting temperatures down to -55° Celsius and pressures of 300 atmospheres.

Under such harsh conditions, the microbes would not be able to grow and reproduce, but they would still be able to repair any molecular damage, keeping themselves viable for more than a thousand centuries, the team says. “It is not life as we generally think about it,” says Rohde. “[They] are just sitting there surviving, hoping that the ice will melt.”

In 2012, scientists germinated flowers from a handful of 32,000 year old seeds excavated from the Siberian tundra. [In 2014], researchers hatched 700-year old eggs from the bottom of a Minnesota lake, while another team resuscitated an Antarctic moss that had been frozen since the time of King Arthur. Bacteria, however, are the uncontested masters of cryogenics—one bug, at least, was alive and kicking after 8 million years of suspended animation.

Conclusion

You can plausibly (if highly unlikely) have either a virus or a bacteria from dinosaur times survive until today and infect humans. A virus will probably spread faster but is far less likely to be able to infect us (but likely to be extremely dangerous if it did). A bacteria is more likely to be able to infect both humans and dinosaurs but unlikely to spread so fast unless some other mechanism (such as fleas for the Black Death) gets involved.

Your best mechanism is probably a deep-frozen infected dinosaur that is released from being frozen by global warming. If it entered melt water the dinosaur corpse could easily be washed down and enter a water reservoir infecting anyone who drinks from it.

The main obstacle is still going to be having them "compatible" with humans after millions of years of evolution. I think you're just going to have to "hand wave" that bit by saying it just happened - unlucky coincidence.

$\begingroup$To be honest, I think the biggest obstacle to realism in this scenario isn't compatibility with humans. As you point out, both viral and bacterial pathogens do jump species barriers, even from dinosaurs (i.e. birds), now and again. In contrast, the base assumption - that a single kind of pathogen was able to wipe out an entire group of animals as diverse as the dinosaurs - represents a scenario virtually unknown in real life. So I wouldn't worry about this.$\endgroup$
– OlleJan 17 at 15:40

The genomes of most complex organisms are full of old viruses, some incredibly old.

A virus is so simple an entity that it consists of little more than a shortish strand of DNA and some accessory proteins that shield it against the elements and help it get into a host cell. When it does get into a host cell, it hijacks the molecular machinery therein to 1) copy its own DNA, and 2) make more virus proteins using instructions contained in its DNA. Virus DNA and virus proteins self-assemble into new viruses, and off we go again.

But there is an alternate path for a virus, and that is to copy/paste its DNA into the DNA of the host cell, forming what's known as a provirus. When the host cell divides, copying its DNA into two daughter cells, the virus DNA is copied along with it. If the provirus ends up a sperm or egg cell, it can be transmitted into the offspring too - and now it's in every cell of the offspring. In this way the virus can lie dormant inside its host's genome, up to and including every genome of an entire host population, potentially forever. So the average animal genome is full of old proviruses. Most are decayed and no longer produce functional virus particles, but some do, even after many millions of years. They can even become symbiotically integrated into the host's biology, but that's a different story.

Birds are the only kind of dinosaurs that have survived to the present day. Suppose that, in your story, this is because they were the only dinosaurs that evolved resistance to the virus - so the virus no longer makes birds ill, but it remains integrated in their genomes and can produce functional virus particles.

Next, suppose some in species of bird - take your pick, depending on whether you want patient zero for your plague to be a chicken farmer, a duck hunter or a pigeon pest control worker - the old virus suddenly mutates into a more virulent version, that evades the birds' immune system, actively replicates and makes the host bird population quite ill. And next, suppose the virus mutates again, making it able to cross over into humans.

Finally, something that might be good to know: the idea that the dinosaurs perished due to an asteroid impact is quite well-established, mostly because we have a crater and other extensive geological evidence that an impact did in fact occur. So for your story, you either need to write the Chicxulub impact out of history, or make it so that it was only a partial contributing factor to the dinosaurs' decline, the main/final killer being the plague.

Evolution happend pretty quickly even quicklier for single cell organism with a very fast living cycle.

Let's say your thing is a deadly bacteria.
It's have spread fast to most species (on land and in the ocean)at dinosaur time. In the very deep of an ocean a speecie seems to take advantage of this bacteria and the bacteria can thrive in the organism of this animal. This organism was very adapt to its environnment so it didn't evolve for millions years, the bacteria was very adapt to this animal so it didn't evolve either. Every deep see species evolved and got resistance to this bacteria. All of this was acting as a barrier between the surface and this deadly bacteria.
So this bacteria was contain safely in this animal in the deep water.

With intensive fishing humans have been harvesting fish deeper and deeper and one day, one fisherman, harvested one of this unknown animal with the deadly bacteria in it.

Since a plague can lie dormant for millions of years (and hence be still as deadly to humans as the day it was first created), then we only need to consider a couple factors:

As a prerequisite, during the dinos, any dino went deep into the Earth and carried the plague there with them. Alternatively, a dino traveled far north where the plague got buried with the dino deep in the arctic or antarctic permafrost deep in a glacier. A third option is that a hurricane (or even a giant volcanic eruption) caught some of the plague and deposited it either deep underground and buried it, or threw it up into the permafrost regions.

After the dinos die out from this plague, enough time elapses for the plague to also die out on the surface, or, say that a sunspot scorches the Earth and burns it up. However, deep underground or deep in the polar permafrost, the plague still lies dormant, waiting to be uncovered.

The ways that this plague could then be released include many possibilities including that some centuries or millennia later:

A great earthquake rocks the earth and uncovers the plague releasing it into the atmosphere, where it multiplies once again.

A sunspot melts the permafrost and uncovers the plague releasing it into the atmosphere, where it multiplies once again.

Scientists stationed in the polar regions uncover the plague while retrieving ice core samples, releasing it into the atmosphere, where it multiplies once again.

A great volcanic explosion erupts, carrying with it the deeply buried plague releasing it into the atmosphere, where it multiplies once again.

An asteroid hits the earth, causing a great upheaval of earth or ice, uncovering the plague and releasing it into the atmosphere, where it multiplies once again.

A researcher finds a previously unknown grove of prehistoric forest in which lives mosquitos, ticks and/or fleas that carry the virus in their digestive tract. The researcher brings one or more if these blood sucking parasites back to civilization where the disease spreads.

You can even claim this as based on a true story. In 1994, a tree known only through the fossil records, the Wollemi Pine was discovered in Australia.

If there are trees from that era, why couldn’t there be insects, bacteria and viruses from that era?

Its only known home is a tiny 5,000 square metre relic grove of
prehistoric rainforest in the 500,000-hectare park. So far only 23
adults and 16 juveniles have been found, making it also one of the
world's rarest plants.

Some species have lived since the dinosaur ages and have hardly evolved in the meantime. A species like that could having a symbiotic relationship with a virus or bacteria and changes in its diet or environment could cause it to start leaking this symbiont to its surroundings. Make your species one of fish to allow the disease to brood under the surface for a few years before any humans notice it. Perhaps the oceans could be affected somehow prompting humans to look for the cause, which triggers the above-water outbreak.

When the dinosaurs died the plague went with them but maybe the plague doesn't come from the earth but instead from space!

We all know that the dinosaurs were killed by a big old asteroid right? Well what if the asteroid wasn't as big as we thought and had some frozen bio material inside which was basically the plague that killed most of the dinosaurs (if all died we wouldn't have birds after all), and when the dinosaurs were almost extinct the plague disappeared from the earth without leaving a trace or the remaining dinosaurs just became immune to it and that immunology is still present on most fauna on earth but not on humans.

After 66 million years later another asteroid with the same plague comes (could be an asteroid that comes nearby the earth every 66 millions years and a small part comes out) and crashes letting the plague loose ready to infect humans because they are not immune like almost every animal in the planet.

It's almost unheard-of rare for things to even transmit within closely related species like mammals.

A thing most people don't seem to understand: most of the really bad human diseases aren't human diseases at all. They're zoonotic diseases that crossed over. The reason they're so bad is we haven't spent millions of years evolving against them.

That might sound like support for that this is possible. It very much means the opposite.

Any given successful species has thousands of diseases, and we've interacted extensively with hundreds of successful species. It is likely that we've interacted directly with more than a million diseases specialized to foreign organisms.

And we can name the number of them that crossed over to us without leaving our fingers and toes. Many of those closely related. All from very, very closely related species.

Humans get mice and cow diseases? Sure. But you'll never see a human get a lobster disease, a beetle disease, a lizard disease.

Why is that?

Because crossing the species barrier is incredibly unlikely, and the further apart the species, the less likely it is.

People tend to forget how biologically close humans are to mice and cows. They're the neighbors of the primates. The mammal class appeared 200my ago, but, most of us are within 50my of one another. The reptiles split away from us 320my ago.

Even within 50my, we've taken on fewer than 30 diseases from all the other species put together. The number of coincidences needed is just astronomical.

.

There are also no known skin-color selective diseases (all known skin color affected syndromes are lifestyle bound.)

I've got to say, that idea makes me really uncomfortable. It sounds like you're going to have a skin color die out. Gross.

.

If you want an ancient disease to cause problems, have it go after the blue-green algae in the ocean. They're the bulk of our oxygen, and still roughly the same creatures that existed back then.

$\begingroup$Welcome to Worldbuilding, John Haugeland! If you have a moment, please take the tour and visit the help center to learn more about the site. You may also find Worldbuilding Meta and The Sandbox useful. Here is a meta post on the culture and style of Worldbuilding.SE, just to help you understand our scope and methods, and how we do things here. Have fun!$\endgroup$
– GryphonJan 17 at 21:36

Some wise guy tried the Jurrasic Park sequence to try to bring back some dinosaurs via DNA from blood in amber. The dino DNA was done for but the plague spores were viable and attempting the process revived them.

The ability to defend against it is linked to overproduction of IgE antibodies (which normally leads to allergies).

Something to consider. we may be one species, but we rely on sooo many others.

A Single micro organism would not be able to wipe out the broad range of species when the dinosaurs died out. At least not directly. It might hit a handful of species but is highly unlikely to hit all of them.

However, lots of organisms rely on a range of bacteria in the intestines to absorb nutrients from our food. That might be a plausible vector to hit a broad range of species hard enough to drive extinction level effects.

To select randomly from commonly know bacteria in the intestinal tracts, lets use E. coli. Suppose that within some preserved stuff from around the time of the KT event an old strain of E. coli has gone into a dormant form. Newly awoken, this ancient E. coli begins spreading, first by getting on some edible stuff in a third world country, where human waste is used as fertilizer. A person eats it, waking the dormant bacteria. The dormant bacteria begins dominating all of the other gut bacteria by consuming more sugars and nutrients. This will sicken or even kill the host, but the process will take time, long enough to spread to the rest of the area. Livestock will also take on this bacteria, and since livestock dung is also a popular fertilizer, the bacteria will spread even more.

Some would think a simple antibiotic will take care of this, but consider that E. coli is tough, and if we are talking about an ancient strain, it may already be resistant to fungal based antibiotics. Remember this strain evolved in a different kind of environment that we really don't know all that much about. Boost this in your story and you could have a strain of E. coli that has broad resistance like some of the Super Bugs modern hospitals are getting worried about.

So you have a super bacteria that out competes other gut bacteria, starving the host, but it's going on in places where people dying from malnutrition is common enough that no one gets surprised by the first several thousand fatalities. This allows the bacteria to spread quite far before it shows up on anyone's radar. It's not a direct infection, it simply overwhelms and outcompetes the current range of bacteria that a large number of species rely on in their intestines, starving them to death. Each species that gets hit hard is going to cause ripples in the web of dependencies. After a certain point you could get cascading effects. That could perhaps cause mass extinctions where the survivors are already pretty adaptable species that don't rely as heavily on gut bacteria.

Most likely the plague would have to produce a toxin or other harmful factor that can affect cell motility. If this was the case then mammals in general along with reptiles, fish, and birds wold be affected as well humans would not be the only ones affected but rather ally animal life would be affected except maybe the invertebrate species. You suggest a 30% immunity from the plague depending on race this wouldn't make any scientific sense except that isolated populations might survive the whole plague idea would have to be scraped.

Questions to consider:

Is it a viral or bacterial plague?

How is it transmitted from person to person?

How does this plagues immunity work?

What caused the plague in the first place?
I hope these questions help you answer your question and also see it in a different light.

A) Become stagnant for millions of years and disappear from history?
This question have several answer that depend if the plague is viral or from prokaryote(bacteria or archaea) origins.
- if it is viral:

The easier answer, a virus is a genetic code program to take control of hosts cell to produce other virus (often killing the hosts cell) (precision that is what we call lytic cycle, they are also able sometimes to insert themself in genomes in a dormant state for decades centry ect... sometimes even loosing they identity actually placenta in mammals is possible due to a viral genes that have been domesticated.)

Second possibility virus are encapsulated in a capside which is otfen compose of protein and suger (in chemical sens) some virus can survive out of their hosts in their capside. Life in capside higly depend on the virus and on the environment. may be the virus was in higly favorable environment for survivavle waining for a new host frozen and free by global warming, inside amber etc...

Prokaryotic microbes: In this case the most plausible is what is call starvation form which are form that microbes tkae when the environment is higly deleterious or lack ressource, you may have heard of microbe surviving in space, here you are they are in starvation form. Again there ability to cam back to life if higly dependant on the type of microbe and the environement...

the hosts as "Rana sylvatica" evolved to survived extrem winter by being able to froze and then come back may be a small infected dinosaurs was taken inside a glace and free again due to global warming.

B) Only to return again in modern day to wreck havoc for a second time. Whether it be by the hands of a scientist or through some other event. It returns and hits hard.

higly depend on A.

C) Be deadly to humans despite being tens of millions of years old?
This is the more problematic, and the less likely due to several factor. You may just forget the plague agent specificaly infect human has is very likely it is not able to target human cell because they are different to dinosaurs cells. at least at the begining you can expect this to be higly inefective but may be the difference is not that high and a few generation may suffice to adapt, or Lateral gene transfer may happend the plague agent stealing gene or plasmid used by other microbe virus to infect cells.

It may start to infect birds or lizards and them pass to human because they are geneticaly closer to dinosaurs.

It is possible that this virus use a toxine stoping cell mobility and then paralysing immune system leading to host dying to others infections. One advantage is that it may so ancien that our immune system will have a really hard time just to recognize it.

With the toxine hypothesys it may be not necessary for the plague agent to specificaly infect human cells may just being in you digestive track will be enough.

b) The plague reemerges naturally in small pockets from fossils exposed to the surface via weathering. An evil scientist investigates rumors of mass deaths on the other side of the world in remote villages, etc. and discovers what is going on. The scientist then decides to harvest this plague as a biological weapon to use at their discretion.

c) I feel like the hardest part to answer is this. A plagues tend to be very species specific. So it is unlikely it would have been infectious to all dinosaurs, and its even less likely that humans would be able to catch it. Since a virus is basically ruled out, it might make more sense for this to be a bacteria. Maybe it is a very fine, deadly bacteria that is airborne, immune to antibiotics (since it is so ancient and maybe predates antibiotics, or is just immune for some other reason), and produces a toxic substance which kills creatures. Like too much acid, cyanide, etc.

In Greg Bear's books Darwin's Radio and Darwin's Children, certain viruses that had hitched a ride within our genetic code had broken out of our genes and were wrecking hanbok. For your scenario, maybe the dino-killing virus had, at the same time, used our ancient mammalian ancestors as a reservoir species (like bats can be for Ebola). At some point, the virus inserted itself in the genetic code of or ancestor. Now, because of some stressor caused by modern life, this virus has been released from our genetic code, adapted to be able to infect us!

Thank you for your interest in this question.
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